1. Field of the Invention
This invention relates to optical systems of convertible focal length for increase of the focal length by putting a negative lens system in the rear of a positive lens system.
2. Description of the Prior Art
Conventionally, in order to convert the focal length, in most cases, it has been the common practice to put what is called the rear attachment lens, or rear conversion lens in the rear of a master lens. The principle of conversion of the focal length is depicted in FIG. 1. That is, when a negative lens system f' having a negative refractive power is arranged in the rear of a positive lens system f having a positive refractive power, as the distance from the front principal point H to rear principal point H' of the negative lens system, or the principal point interval HH' is generally of positive value, the attachment of the negative lens system f' causes image shift, from a position O for the positive lens system alone, rearwards to a position O'. This is because the negative lens system f' in most cases takes the form that the front lens group is divergent and the rear lens group is convergent so that the principal point interval HH' is apt to be positive. For this reason, it has been very difficult to achieve the coincidence of the position of an image point resulting from the attachment of the negative lens system which has found its general use in the conventional focal length-convertible systems to the rear of the positive lens system with that of an image point resulting from the positve lens system only.
It is known to put the negative lens system in an intermediate space of the master lens system in converting the focal length of the entire system as in U.S. Pat. No. 4,240,697. As for the attachment of the negative lens system to the rear of the master lens system in converting the focal length of the entire system, mention is made of U.S. Pat. Nos. 4,129,359 and 4,154,508.